Chickens will conceal symptoms of illness until they can't anymore, so she is pretty sick. Have you dewormed her? does she have diarrhea or are her droppings normal? If you haven't dewormed her, you need to asap. If you can get Safeguard suspension from your vet, or Safeguard goat dewormer from your feed store, that will work, it has to be a broad spectrum dewormer. She may have Coccidiosis too, but if you haven't ever dewormed her start with deworming first. She needs to be in a pen...or crate.....where she can have some privacy and warmth and be by herself but ideally still be able to hear and/ or see her friends. If you haven't dewormed the flock, do that also, even if they are not showing symptoms. Safeguard suspension dose is 1 cc per day for a standard size chicken (according to my vet) for three to five days. You can soak breadcrumbs in buttermilk and mix the dewormer in that, and she should eat it, or on chopped up grapes or blueberries or whatever she likes. If she won't eat it, you can slowly dribble it into the side of her beak. I'm not sure about the safeguard goat dewormer dose, but you can search on here and should find it. Most vets will do a fecal check for you, so then you will know if it's parasites and what type you are dealing with, but if you are unable to do that, go ahead and deworm anyway. It shouldn't hurt her. Act quickly though because worms can kill chickens, and so can Cocci.
You should deworm the flock twice a year with a broad spectrum dewormer. We use Levasol, which is safe and effective. Safeguard is popular too.
